In real applications, motors often fail to start up with heavy load because of insufficient torque. There is a need to optimize the utilization of the power source to successfully start the motor and thereby shorten the transition time from stillness to rated speed.
FIG. 1 shows a prior art drive and control circuit for a motor system. In FIG. 1, the drive and control circuit comprises a control circuit 101 and a power stage. The power stage comprises four switches SW1, SW2, SW3 and SW4. The control circuit 101 generates four control signals PWM1, PWM2, PWM3 and PWM4 to respectively control the four switches SW1, SW2, SW3 and SW4. The operation is: when the switches SW1 and SW4 are turned ON by the control signals PWM1 and PWM4, the switches SW2 and SW3 are turned OFF by the control signals PWM2 and PWM3, thus the current flowing through the motor 102 flows in direction a as shown in FIG. 1; when the switches SW2 and SW3 are turned ON by the control signals PWM2 and PWM3, the switches SW1 and SW4 are turned OFF by the control signals PWM1 and PWM4, thus the current flowing through the motor 102 flows in direction b as shown in FIG. 1. By alternatively changing the direction of the current flowing through the motor 102, the motor 102 runs with a fixed direction.
During when the current flowing through the motor 102 flows in direction a, the switch SW4 stays ON, and the switch SW1 is turned ON and OFF at a frequency of 25 kHz. Similarly, during when the current flowing through the motor 102 flows in direction b, the switch SW3 stays ON, and the switch SW2 is also turned ON and OFF at a frequency of 25 kHz.
FIG. 2 shows waveforms of signals in the circuit of FIG. 1. In FIG. 2, “Vcosc” represents a sawtooth signal and “Vth” represents a reference signal. “Speed” represents a pulse signal, and the frequency of the pulse signal is proportional to the speed of the motor. The sawtooth signal Vcosc is compared with the reference signal Vth to generate a control signal PWM. Persons of ordinary skill in the art should know that the control signal PWM is corresponding to the control signals PWM1 or PWM2 in FIG. 1. When the sawtooth signal Vcosc is fixed, the pulse width of the control signal PWM is determined by the reference signal Vth. As shown in FIG. 2, when the reference signal Vth is lower than the sawtooth signal Vcosc, the duty cycle of the control signal PWM could even be 100%, thus resulting in a maximum motor torque, and thereby resulting in a maximum motor speed.
In real applications, the motor is expected to start up quickly so as to shorten the transition time from stillness to rated speed. Thus the duty cycle of the control signal PWM should be sufficient enough to achieve a maximum torque when the motor starts up.
FIG. 3 shows the waveforms of signals in a conventional system with a control signal PWM having variable duty cycle. When the reference signal Vth is lower than the sawtooth signal Vcosc in the startup interval in FIG. 3, the control signal PWM has a 100% duty cycle, thereby the motor starts up with a high torque and gets started up easily and quickly. After the startup interval, the motor enters a steady state. The duty cycle of the control signal PWM changes to a set value determined by the changed reference signal Vth, and the speed of the motor is proportional to the duty cycle of the control signal PWM.
FIG. 4 shows the waveforms of signals in a conventional system with a control signal PWM having a fixed duty cycle. As shown in FIG. 4, the duty cycle of the control signal PWM starts directly from a set value determined by the constant reference signal Vth. Thereby the motor has a much smaller torque than that in FIG. 3, resulting in a much longer transition time from stillness to rated speed. If started up with a heavy load, the motor may fail to start up due to insufficient torque.